1. Field of the Invention
The present invention relates to image processing and in particular to halftoning of image data. More specifically, the present invention relates to multi-resolution halftoning of image data by performing a low resolution halftoning process for black image data and a high resolution halftoning process for color image data.
2. Description of the Related Art
Image processing to print images from a computer generally includes processes by a print driver on a host computer side and processes by a print control board on a printer side. The print driver includes processes for rasterization, halftoning, and data compression and transfer to the printer. The print control board includes processes for data expansion (sometimes including indexing) and smoothing. Because of processor limitations (speed, memory, etc.), it has been challenging to obtain a good balance between each of these processes to obtain good printing performance (image processing speed) and a good quality image. More particularly, it has been difficult to achieve high performance printing with good smoothness in gradation areas as well as good text quality.
Rasterization generally refers to a process of a graphics device interface converting graphics image data into rasterized image data to be utilized in printing the displayed image. Conventionally, rasterization is performed by processing an image at display resolution (typically 72 dpi (dots per inch)) to extract red (R), green (G) and blue (B) values for each pixel and to render the data into a rasterized image at print resolution which may be either low resolution (e.g. 300 dpi) or high resolution (e.g. 600 dpi). The rasterization process processes both black image data and color image data with the same rasterization process, i.e. either a low resolution rasterization process for both black image data and color image data or a high resolution rasterization process for both. Performing a high resolution rasterization process provides for a higher quality image. However, the high resolution rasterization process significantly increases processing time, as well as processing time for downstream processes, thereby providing for a lower performance. Thus, to provide for increased performance, a low resolution rasterization process is preferred.
The RGB values obtained in the rasterization process are processed to obtain continuous tone values of cyan (C), magenta (M), yellow (Y) and black (K), which are values associated with printers. In halftoning, the continuous tone CMYK values for the image data are subjected to a process that determines whether or not an ink droplet is to be ejected at a particular pixel location. The halftoning process generally comprises comparing an input image data value to a threshold value and printing a dot if the value is above the threshold or not printing a dot if the value is below the threshold. This type of halftoning is commonly referred to as bi-level halftoning.
Another type of halftoning process is multi-level halftoning. Multi-level halftoning is similar to bi-level halftoning, except that additional threshold comparison tests are performed. For instance, input image values may range from 0 to 255. The input value may be compared to a threshold of 128 (mid-point of the range). If the input value is above 128, then a second threshold test may be performed for a range of 128 to 255 with a threshold of 192 (mid-point of the second range). Further threshold tests may continue to be performed until the difference between the image data value and the threshold value falls within a desired limit. Of course, additional iterations could also be performed in the same manner if the initial threshold test determines that the input value is below the initial threshold. Multi-level halftoning provides for increased image quality over bi-level halftoning, but increases processing time due to the multiple iterations.
Halftoning processes may also include an error diffusion process. The error diffusion process somewhat compensates for errors that result from the threshold comparison between the input image value and the threshold value. The compensation is provided by a process that diffuses the errors to surrounding pixels of the subject pixel.
Conventionally, the halftoning process is performed in either high resolution or in low resolution, the former resulting in a higher quality image and the latter resulting for a lower quality image. Whether the halftoning process is a high resolution process or a low resolution process is generally determined by the resolution of the image data input to the halftoning process. Thus, if the image is rasterized in low resolution and the low resolution data is subjected to halftoning, the halftoning process is low resolution. Similarly, if the image is rasterized in high resolution and the high resolution data is subjected to halftoning, the halftoning process is high resolution. Of course, a process to expand low resolution rasterized data into high resolution data prior to being subjected to the halftoning process could also be performed, thereby resulting in high resolution halftoning.
Conventionally, a single halftoning process is generally performed on the image data. That is, both black image data and color image data are subjected to the same halftoning process. Therefore, if a high resolution image is desired, high resolution halftoning is performed and both the black image data and the color image data are subjected to the same high resolution halftoning process. However, high resolution halftoning increases the processing time, thereby resulting in lower performance.
As a result of the rasterization and halftoning processes, low resolution rasterization and low resolution halftoning increase performance but result in a low quality image. Conversely, high resolution rasterization and high resolution halftoning result in a good quality image, but increased processing time, i.e. low performance.
As previously stated, it is possible to perform a low resolution rasterization process and expand the low resolution image data into high resolution image data prior to being subjected to the halftoning process. This combination reduces the processing time somewhat, as compared to a high resolution rasterization/high resolution halftoning combination and generally provides for good smoothness in gradation areas, but text quality remains low.
It has also been considered to utilize low resolution rasterization and low resolution multi-level halftoning, combined with indexing and smoothing processes performed in the printer. With this process, the print driver performs low resolution rasterization and low resolution halftoning, and the low resolution data is transferred to the printer, together with index pattern and key information. The index pattern and key are utilized by the printer to expand the low resolution image data into high resolution data. This combination helps to increase performance by allowing less data (low resolution data) to be transmitted to the printer and also provides better smoothness in gradation areas than bi-level halftoning. However, this process results in a lower quality image than would otherwise result from performing high resolution halftoning.
It has further been considered to combine low resolution rasterization and low resolution halftoning, coupled with expansion of the low resolution data to high resolution data (without an index pattern and key) and smoothing in the printer control board. This combination increases performance by providing for a smaller data transfer size and results in good quality text and solid images, but details in the gradation areas are lost.
As can readily be seen from the foregoing, it has been difficult to obtain an image process that provides a good balance of high performance coupled with good quality text and good details in gradation areas.
The present invention addresses the foregoing by performing multi-resolution halftoning in which black image data and color image data are halftoned with different halftoning processes, each corresponding to different resolutions, and performing image smoothing in the printer for black image data so as to obtain high quality for both text and gradation areas, together with good performance.
Accordingly, in one aspect the invention is image processing by rasterization of image data, determining whether the image data corresponds to black image data or color image data, performing a halftoning process of a first resolution on the black image data, and performing a halftoning process of a second and higher resolution on the color image data. The first resolution may be a low resolution and the second resolution may be a high resolution. The halftoned black and color image data is transferred to a printing device, together with a command for the printing device to perform smoothing-processing on the black image data, whereby the printing device outputs an image. The invention may also provide for expanding the black image data to high resolution data in the printing device.
As a result of the foregoing, since the halftoning process is performed separately for black image data and color image data at different resolutions, a higher performance may be achieved for processing of black image data and a high quality image may be achieved for processing of color image data. This is in contrast to conventional methods in which, in order to achieve a high quality image, a high resolution halftoning process is performed on both color and black image data, thereby increasing the processing time. Additionally, performing a smoothing process on the black image data provides for good quality text and good details in gradation areas. Further image quality for the black text can be achieved by the expansion process. Therefore, the problem of obtaining a good balance between performance, text and color image quality and good details in gradation areas may be achieved by the present invention.
The invention may also provide for processing image data according to the foregoing and printing the data with a print head having a faster printing speed for printing black image data than for printing color image data. The faster printing speed of the black image data provides for even further advantages in performance.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.